Never ever refund before you get the item back (unless you haven't asked for it back). Once a buyer has the refund, there is no incentive to return it. Sadly, apart from suing them through court, there is very little you can do to get it returned once you've refunded. If you don't follow the rules that eBay have, they won't help you either. That is, accept the return, buyer returns it, you refund. 

Hopefully they are honest enough to return it. Did you send them a postage label to return it?